The present invention relates to an optical-fiber gyroscope of the Sagnac type having an optical fiber coil and a 3.times.3 coupler comprising three optically coupled fiber optic guides, whereby a first fiber optic guide of the 3.times.3 coupler is connected at one end to a first optical radiation source, whose radiation intensity is detected directly by a radiation detector, and the two other fiber optic guides of the 3.times.3 coupler are connected respectively at one end, to respective ends of the optical fiber coil and, at the other end, to respective radiation detectors.
U.S. Pat. No. 4,440,498 discloses an optical-fiber gyroscope Which can be operated in so-called phase quadrant by using a 3.times.3 coupler. Also, German Application P38 05 904.5 discloses an optical-fiber gyroscope with which fluctuations in the radiation source, as well as the coupler characteristics, can be detected with the aid of a so-called monitoring diode. See FIG. 1 of the drawings of the present application which shows a diagram of the optical fiber gyroscope according to the German application. With reference to FlG. 1, in the evaluation of signals using diodes 4 and 5, using two monitoring diodes 7 and 6 disposed, respectively, coupled to the radiation source 3, and to the central channel 1 of the 3.times.3 coupler 2, fluctuations in the radiant power of the light source 3 can be detected and fluctuations in the coupling length of the 3.times.3 coupler, which, for example, are conditional on the thermal expansion of the coupler, can also be detected. With this device, however, one is only able to detect the common variation in the three coupling lengths among the three wave guides inside the 3.times. 3 coupler, that is .DELTA.(K.sub.12 L)= K.sub.12 .DELTA.L, .DELTA.(K.sub.13 L)=K.sub.13 .DELTA.L and .DELTA.(K.sub.23 L)=K.sub.23 .DELTA.L(K.sub.ij =coupling factor between the fiber optic guides i and j, L =coupling length). Changes, which result from environmental influences such as pressure, acceleration or temperature, in the desired symmetry of the coupling factors K.sub.13 =K.sub.23 =K.sub.12, however, are not considered or determined.